A synthetic paper obtained from a starting material mainly containing a synthetic resin has been developed for various applications in view of its superior water resistance, surface gloss, printability of a smooth surface and the like. Particularly, a polyester resin represented by polyethylene terephthalate has high heat resistance and high rigidity among the starting materials for synthetic paper, and has been employed in a wider range of uses. For example, utilizing the low dielectric property derived from a porous structure, it has been used as an insulating material for electric motors (JP-A-9-149576). In addition, utilizing the cushioning property due to the porous structure, it has been used as various printing films for heat transfer printing (U.S. Pat. No. 4,912,085).
For the production of a porous polyester film to be used for such applications, there have been known a method comprising adding inorganic fine particles to a polyester resin, followed by stretching to form voids around the particles, and a method comprising mixing a polyester resin with a thermoplastic resin incompatible with the polyester resin and the like to allow for dispersion of the thermoplastic resin in particles in the polyester resin, followed by stretching to form voids around the particles. Particularly, the latter has been widely used since the film can be made lighter.
The thermoplastic resin (hereinafter also referred to as a void-forming agent), to be used for this void-forming and which is incompatible with the polyester resin, includes polyolefin resin represented by polypropylene and polymethyl pentene (JP-A-49-34755), and a polystyrene resin (JP-B-49-2016, JP-B-54-2955).
Of these, a polyolefin resin, particularly polymethylpentene, has superior void-forming capability, thereby allowing formation of voids inside the film with high efficiency. On the other hand, fine dispersion in the polyester resin is difficult and is susceptible to the formation of coarse and non-uniform voids, which in turn results in lower flexibility (i.e., following capability of a film upon deformation) of a film, to markedly degrade the handling property of the film. To be specific, difficult punch cutting, poor performance during being transported, and poor machine insertion applicability (mounting property) may be found when the film is used as an insulating material for electric motors. These problems are solved only when the porosity of the film is limited to a certain level, which requests sacrificing the properties (e.g., cushioning property, low dielectric property and light weight) derived from a porous structure.
The degraded handling property poses problems when used as the above-mentioned electric insulating material, and in terms of transportability (failure in paper supply, jamming of paper and the like), printing failure on wrinkles and the like in high speed handling for various printing methods. To solve these problems, it is again necessary to limit the porosity, sacrificing the properties (e.g., cushioning property, low dielectric property and light weight) derived from a porous structure.
In contrast, when a polystyrene resin is used as a void-forming agent, fine dispersion in the polyester resin is more easily achieved than when a polyolefin resin is used, and the handling property of a film becomes strikingly superior. However, a polystyrene resin has a lower void-forming capability than a polyolefin resin, and the properties (e.g., cushioning property, low dielectric property and light weight) derived from a porous structure cannot be fully expressed.
There have been made some attempts to provide for fine dispersion of a polyolefin resin (e.g., polymethylpentene) in a polyester resin. For example, the use of a surfactant (JP-B-7-17779), polyethylene glycol (JP-A-2-235942), polyether ester copolymer (JP-A-4-264141) and the like has been proposed.
The proposed use certainly reduces coarse and non-uniform voids and improve flexibility of a film to a certain degree, somewhat improving handling property of a film. However, the method including the use of a surfactant causes high adhesion between a polyester resin and a polyolefin resin (e.g., polymethylpentene), which inhibits formation of voids upon stretching. In addition, the method including the use of polyethylene glycol and a polyether ester copolymer significantly impairs the rigidity (nerve) inherently possessed by a polyester film, because these components act as plasticizers to the polyester resin. The lower rigidity conversely degrades handling property of a film. Moreover, the ether component is easily oxidized to cause coloring of a film and sometimes promotes degradation of a polyester resin.
According to the prior art technique as described above, a film having both the superior properties (e.g., cushioning property, low dielectric property and light weight) derived from a porous structure and good handling property has not been obtained.